Seismic data acquisition and processing techniques are used to generate a profile (image) of a geophysical structure (subsurface) of the strata underlying the land surface or seafloor. Among other things, seismic data acquisition involves the generation of acoustic waves, the collection of reflected/refracted versions of those acoustic waves, and processing the collected seismic data to generate the image. This image does not necessarily provide an accurate location for oil and gas reservoirs, but it may suggest, to those trained in the field, the presence or absence of oil and/or gas reservoirs. Thus, providing an improved image of the subsurface in a shorter period of time is an ongoing process in the field of seismic surveying.
A configuration for achieving land seismic data is illustrated in FIG. 1. FIG. 1 shows a system 100 that includes plural receivers 102 positioned over an area 104 of a subsurface to be explored and in contact with, or below the surface 106 of, the ground. A number of dedicated seismic sources 108 are also placed on the surface 106 in an area 110, in a vicinity of the area 104 of the receivers 102. Note that a dedicated seismic source is defined as a device built by man with the main purpose of generating seismic waves to be used for a seismic survey. Alternatively, dedicated seismic sources 108 may be buried under surface 106. A central recording device 112 is connected to the plurality of receivers 102 and placed, for example, in a station/truck 114. Each dedicated seismic source 108 can be composed of a variable number of vibrators, typically between one and five, and can include a local controller 116. A central controller 118 can be provided to coordinate the shooting times of sources 108. A global positioning system (GPS) 120 can be used to time-correlate shooting of the dedicated seismic sources 108 and the recordings of the receivers 102.
With this configuration, dedicated seismic sources 108 are controlled to intentionally generate seismic waves, and the plurality of receivers 102 records waves reflected by oil and/or gas reservoirs and other structures.
However, a typical problem encountered with this kind of seismic acquisition system is the presence of obstacles above the subsurface of interest. More specifically, as illustrated in FIG. 2, acquisition system 200 includes plural seismic receivers 202 distributed above the subsurface 220 of interest. One or more dedicated sources 208 may travel above the subsurface 220 or they may be buried at permanent location to generate seismic waves. However, various obstacles 230 and 240 may be present above subsurface 220 and they may prevent dedicated seismic sources 208 to shoot from those locations. As is known in the art, the dedicated seismic sources follow a pre-plotted track and shoot seismic waves at locations typically corresponding to a grid. The presence of obstacles 230 and 240 disturb the grid and limit available shot locations, which results in poor recorded seismic data for some portions of the subsurface.
Obstacle 230 is illustrated in FIG. 2 as a factory having various buildings 232, while obstacle 240 is illustrated to be a residential area having houses 242 and streets 244. While seismic receivers may be distributed within these obstacles, it is very unlikely that the dedicated seismic sources would be allowed to enter them. This is not only because the owners of the obstacles will not want dedicated seismic sources (typically big trucks with an oscillating plate that generates seismic waves) to disturb their properties, but also because state and county regulations usually prevent such activity in residential and industrial areas.
While the obstacles shown in FIG. 2 are man-made, it is also possible to have naturally occurring obstacles, such as hills, mountains, canyons, abysses, ravines, etc., in which it is not possible to deploy dedicated seismic sources.
Thus, there is a need to obtain seismic data for locations inaccessible to dedicated seismic sources to acquire a better image of the surveyed subsurface.